Process for treating sulphur-containing spent liquor using multi-stage carbonization

ABSTRACT

To recover the sodium-sulphur compounds following sodium-based pulp processing, e.g. the ASAM process or basic or acidic sodium sulphite processes, the waste liquor is burnt in a lye burning vat (1) with liquid slag extraction and the exhaust gases are subjected to multi-stage purification with dry fly-ash recovery and the gaseous sulphur compounds are also separated in a multi-stage washing process. The water-soluble components of the fly-ash are taken to the waste liquor to be burnt and thus the sulphur to be recovered in the slag is increased. The slag is dissolved in water and the dissolved sodium compounds are converted by multi-stage carbonisation with a part of the purified flue gas into sodium bicarbonate and H 2  S. The H 2  S is burnt and finally converted into sodium sulphite so that the desired lye for pulping cellulose can be made up from sodium sulphite and carbonate.

PROCESS FOR TREATING SULPHUR-CONTAINING SPENT LIQUOR USING MULTI-STAGECARBONIZATION

The present invention relates to a process for converting spent liquidcontaining sodium sulfite and carbonate with sulfate and thiosulfateimpurities into cooking liquor containing sodium sulfite and sodiumcarbonate for an sodium (Na)-based cellulose pulping process or Na-basedpulp digestion process, such as, for example, the ASAM process oralkaline or acidic sodium sulfite processes, in which the spent liquoris burned in a liquor burning boiler with liquid slag extraction and amultistage waste gas purification with recovery of the sodium sulfurcompound. The waste gases leaving the liquor boiler are first freed ofdust in a dry process and are subsequently in one stage, preferably inat least two stages, washed with different washing liquids. Theseparated dust, in particular the separated Na₂ SO₄ is mixed with theliquor to be burned and the liquid slag from the liquor burning boileris dissolved in water. The dissolved sodium compounds, in particular theNa₂ S formed, is converted by carbonation with a portion of the purifiedwaste gas to form NaHCO₃, NA₂ CO, and NaHS.

The ASAM process (Alkaline Sulfite Process with Anthraquinone andMethanol addition) is a further development of the neutral or alkalinesulfite process known and used in the industry for centuries. The trueinnovation in the ASAM process is the addition of methanol to thepulping solution. In comparison to the sulfite and to the sulfateprocesses, the ASAM process has the advantage that during the digestion,gaseous sulfur compounds do not develop and that the chemical pulp canbe bleached to the highest degrees of whiteness without the use ofchlorine-containing bleaching agents. Since when using chlorine-freebleaching agents in the digestion as well as also in the variousbleaching stages, exclusively sodium is present as the base, theresulting bleaching tower spent liquor can be processed during the pulpwashing stage together with the spent liquors from the digester house.The alkali used in the various bleaching stages can be recovered and thepossibility is given of largely closing the water cycle of the factory.

With reference to the prior art, several prior art processes will bedescribed.

Austrian Patent No. AT-B 351 359 discloses the removal of NaCI from thedigestion liquid without including the solid residues of the waste gaspurification.

European Patent Publication No. EP-B1 223 821 discloses a pyrolysisprocess for the spent liquor in which a portion of the combustiblecomponents resulting in the pyrolysis is burned and the inorganicresidues are present in the molten state and are quenched. In thisprocess as well, the solid residues from the waste gas purification arenot introduced into the smelt.

European Patent Publication No. EP-A1 538 576 discloses the carbonationof the green liquor with CO₂ -containing gas, such as for examplepurified waste gas or gas or from the causticizing stage with theformation of NaHCO₃ and the release of H₂ S. The concentration of H₂ Sis decreased by the nitrogen contained in these gases leading to anincrease in the gas volumes to be treated so that large quantities ofgas must be supplied to the liquor boiler or must be burned in the H₂ Smuffle. Moreover, the conversion of the NaHCO₃ formed with NaHS to formNa₂ CO₃ and H₂ S leads to an increase of the pH-value and a decrease ofthe partial pressure of H₂ S in the solution as well as to a highrequirement of stripping gas as a result.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new and improvedprocess for converting sodium sulfate in which the disadvantages of theprior art are avoided.

The invention is based on the task of addressing the problems and ofclosing the recovery of the chemicals via the cycle of conversion ofspent liquor to cooking liquor. The recovery installations have two mainfunctions: the recovery of inorganic pulping chemicals from the spentliquor for the preparation of the cooking liquor as well as theutilization of the energy contained in the organic substance ashigh-pressure vapor. The value of the recovered chemicals exceeds thevalue of the required vapor energy.

The invention solves the task and is characterized thereby that the H₂ Sand CO₂ containing slag (green liquor), flowing off from thecarbonation, is striped in several stages with CO₂ and water vapor andthat after every stripping stage CO₂ is absorbed at pressures of >1 andthe remaining H₂ S gas, after the condensation of water vapor, is burnedin an H₂ S muffle.

Briefly, the process in accordance with the invention is directed toconverting spent liquor containing sodium sulfite and carbonate withsulfate and thiosulfate impurities to sodium sulfite and sodiumcarbonate-containing cooking liquor for an Na-based cellulose pulpingprocess. Particular to Na-based processes are the ASAM process oralkaline or acidic sodium sulfite processes in which the spent liquor isburned in a liquor burning boiler with liquid slag extraction and amultistage waste gas purification with recovery of the sodium sulfurcompounds, and waste gases leaving the liquor boiler are first freed ofdust in a dry process and subsequently in at least one stage are washedwith different washing liquids, and the separated dust includingseparated Na₂ SO₄, is mixed with the liquor to be burned, and the liquidslag from the liquid burning boiler is dissolved in water and thedissolved sodium compounds including the Na₂ S formed, are converted bysodium compounds including the Na₂ S formed, are converted bycarbonation with a portion of the purified waste gas to form NaHCO₃, Na₂CO₃ and NaHS. In such a process, the invention comprising the steps ofstripping the H₂ S and CO₂ -containing slag flowing from the carbonationstage with CO₂ and water vapor in several stages such that after eachstripping stage, CO₂ is absorbed at pressures greater than 1 bar, andburning the remaining H₂ S gas after the condensation of water vapor inan H₂ S muffle.

The SO₂ and H₂ S-containing waste gases of the liquor burning boiler,freed of dust in a dry process, may be washed in a first washing stageat a pH of 6 to 7 with an Na₂ SO₃ solution to form NaHSO₃, theodor-intensive H₂ S and mercaptan-containing waste gases flowing offfrom the first washing stage are then washed in a second washing stageat a pH greater than 7 to form Na₂ SO₄, the Na₂ SO₄ being recycled tothe spent liquor before it is burned, and lastly the NaHSO₃ solutionfrom the first waste gas washing stage of the liquor burning boilershould be directed in least in part to the SO₂ washing stage after themuffle, such that the bisulfate formed is subsequently mixed with thecarbonated sodium compounds at a pH greater than 6 and the Na₂ SO₃formed is used for the liquor as well as for the washing liquid of thefirst washing stage. The odor-intensive H₂ S and mercaptan-containingwaste gases flowing off from the first washing stage are preferablywashed in the second washing stage with H₂ O₂.

In addition, the process may entail stripping the dissolved sodiumcompounds from the smelt in several stages by CO₂ -containing gasessetting free H₂ S, and providing a bicarbonate splitting stage connectedto a last one of the stripping stages to direct CO₂ to the laststripping stage such that the end product is Na₂ CO₃ containing only lowlevels of NaHCO₃ and H₂ S impurities and is suitable for causticizing inorder to obtain NaOH. The stripping of the H₂ S in several stages ispreferably carried out at increasing total pressure and increasedtemperature and fortification with CO₂ to bicarbonate at an increasedtotal pressure is performed between sequential stripping stages.

The basic process in accordance with the invention may further entailintroducing CaO and Ca(OH)₂ into the liquor tank to react with SO₃ toform CaSO₄, utilizing the resulting Na₂ SO₄ and CaSO₄ -containing ashfrom the dry dust-removal stage with the unreacted calcium compounds inthe second washing stage, separating the insoluble calcium compound, andutilizing the concentrated NaOH, Na₂ SO₄ solution in the second washingstage and the consumed washing solution is supplied to the spent liquorto be burned. In this embodiment, flue ash may be separated from theliquor burning boiler in an electrostatic filter and suspended withwater, and sodium carbonate may be added in a clarifier for the completeprecipitation of the calcium at pH-values greater than 6 such thatinsoluble calcium compounds are separated and the thus purified sodiumsulfate solution is again supplied to the spent liquor before the mixtank. Also, the resulting flue ash may be separated in an electrostaticfilter and suspended in water and at pH-values greater than 7, and thenwashed with the addition of H₂ O₂, whereby insoluble calcium compoundsare separated in the form of calcium sulfate and calcium and the thuspurified sodium sulfate solution is again supplied to the spent liquorbefore the mix tank. For this latter embodiment, the calcium fractionsmay be converted with SO₂ at pH-values greater than 5.5 to form calciumsulfate in a flue gas washing stage preceding the alkaline washingstage, and the calcium sulfate thus-formed separated from the sodiumsulfate solution and supplying the solution thus-formed to the spentliquor.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of embodiments of the inventionand are not meant to limit the scope of the invention as encompassed bythe claims.

FIG. 1 shows the overall circuit diagram of the process in accordancewith the invention.

FIG. 2 shows a partial circuit diagram of FIG. 1.

FIG. 3 shows a partial circuit diagram of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the accompanying drawings wherein the same referencenumerals refer to the same or similar elements, as shown in FIG. 1, thespent liquor (black liquor) from the evaporation installation (notshown) together with recycled sodium sulfate from an alkaline flue gaswashing stage 5 and ash from an electrostatic filter 2 are mixed in aliquor tank 7, the flue gas washing stage 5 and the electrostatic filter2 being downstream of the liquor tank 7. The mixed spent liquor, ash andsodium sulfate are supplied from the liquor tank 7 to a liquor burningboiler 1. In or after the flue gas washing stage 5, CASO₄ can already beseparated in the form of an insoluble sediment which reduces the ballastmaterial.

As noted above, the spent liquor is mixed with deposited solid materialsin the liquor tank 7 and, analogously to the sulfate process, burned inthe liquor burning boiler 1 under reducing conditions in a reductionbed. The resulting smelt from the liquor burning process is extracted atthe bottom of the liquor burning chamber of the boiler 1 and theresulting gases are burned with the supply of air via secondary andtertiary air nozzles. The chemical reactions taking place in boiler 1are essentially the following:

1. C+1/2O₂ →CO+H₂

2. CO₂ +C→2CO

3. H₂ O+C→CO+H₂

4. Na₂ SO₄ +2C→Na₂ S+2CO₂

5. Na₂ SO₄ +4C→Na₂ S+4CO

These reactions take place in the reducing zone, the CO-containing wastegases are burned to form CO₂ through the addition of air. A high degreeof reduction is targeted in order to keep the fraction of sodium sulfatelow. During the liquor burning a degree of reduction of about 90% can beattained. A high fraction of liquor sulfur is converted to SO₂ duringthe combustion and consequently set free. The incorporation of sulfurinto the smelt bed is primarily a function of the boiler load as well asof the ratio of sodium to sulfur and is of the order of magnitude of 65%to 85% for the ASAM liquor.

With O₂ values <1 in the waste gas, H₂ S occurs in relatively highconcentrations, therefore an effective O₂ regulation is required inorder to keep the H₂ S formation low. The SO₂ separation takes place ina multistage washing process and about 95% of the accumulating SO₂ iswashed out with sodium sulfite in the first washing stage 4 and isobtained as the product in the form of sodium sulfite and sodiumbisulfate. The residual SO₂ separation takes place in the basic washingstages 5 and H₂ O₂ and the degree to which H₂ S is washed out is alsohigh. Another washing stage is represented by 3 in the which thematerial contained SO₂ is washed with water.

The smelt from the boiler 1 is directed into and dissolved in a tank 6with H₂ O and condensates from the conversion and is furthersubsequently subjected to a decanting process in a decanter 8. Thesteams from the smelt dissolving reaction may be supplied to the liquorburning boiler 1. Sludge or mud is removed from the decanter 8.

The green liquor containing Na₂ S, NaHS, and Na₂ CO₃ is precarbonated,after the decanting stage in the decanter 8, in a multistage wash tankwith CO₂ -containing boiler flue gas with the formation of NaHCO₂. Thefollowing chemical reactions take place in tank 9:

1. 2Na₂ S+CO₂ +H₂ O→2NaHS+Na₂ CO₃

2. 2Na₂ S+2CO₂ +H₂ O→Na₂ CO₃ +2 H₂ S

3. Na₂ CO₃ +CO₂ +H₂ O→2NaHCO₃

This CO₂ absorption preferably takes place at increased or elevatedpressures and low temperature with the pressure being limited toapproximately 1050 to 1060 mbars. The lower temperature limit isdetermined by the solubility of sodium bicarbonate and should be in therange of 30° C. to 45° C. due to possible reactions in which coatingsare formed which can cause disturbances through the precipitation ofsilicates, it is useful to implement this apparatus as a multistagewashing apparatus. To this end, the precarbonated liquor from afterprecarbonation in the multistage washing tank 9, is directed to andfurther carbonated in a carbonation process tank 10 with CO₂ -containingwaste gas from the SO₂ washing process taking place in tank 15succeeding H₂ S muffle 13. A regulated quantity of air is also directedinto the carbonation process tank 10. The following chemical reactionstake place in the carbonation process tank 10:

1. 2 NaHS+H₂ O+CO₂ →Na₂ CO₃ +H₂ S

2. Na₂ CO₃ +CO₂ +H₂ O→2 NaHCO₃

This CO₂ absorption takes place in the same way as during theprecarbonation in the washing process 9 at increased or elevatedpressure and low temperature and it is useful to implement thisapparatus as a multistage apparatus. If sufficient CO₂ from the S_(2O)washing process occurring in tank 15 is made available after a saturator14 positioned intermediate of the H₂ S muffle 13 and the tank 15, it ispossible to omit the precarbonation process. Since the residual oxygencontent of the flue gas after the H₂ S combustion in the muffle 13 issignificantly lower than in the tank flue gas, a lower degree ofoxidation takes place in the washing process in tank 15 from sulfite tosulfate or from hydrogen sulfite to thiosulfate. A regulated quantity ofair is directed through the saturator 14. Possible oxidation processesoccurring in the liquor in tank 15 are:

1. Na₂ SO₃ +1/2O₂ →Na₂ SO₄

2. 4 NaHS+2O₂ +CO₂ →+Na₂ S₂ O₃ +2H₂ S+Na₂ CO₃

The thiosulfate content in the liquor does not interfere with thecooking process in an alkaline environment. It does, however, contributeto an undesirable inactivation of the digestion chemicals.

Rational process control of the H₂ S stripping requires distribution ofthe H₂ S stripping process occurring in member 11 intermediate of thecarbonation process tank 10 and the muffle 13 over several apparatus (asshown most clearly in FIG. 2). Since, due to the two desorptionreactions proceeding simultaneously, sodium carbonate is formed in theparticular stripping stage, it is therefore necessary to carry out anintermediate fortification with CO₂ after each stripping stage in orderto convert the carbonate to bicarbonate again and, consequently, toraise the partial pressure of H₂ S which determines the transfer of thestock. The two desorption reactions with H₂ S and CO₂ thus proceed inmember 11 as follows:

1. 2 Na₂ HS+CO₂ +H₂ O→Na₂ CO₃ +H₂ S

2. 2 NaHCO₃→Na₂ CO_(3+CO) ₂ +H₂ O

3. NaHCO₃ +NaHS→Na₂ CO₃ +H₂ S

If extensive H₂ S stripping is to be achieved, the partial pressure ofH₂ S in the gaseous phase must be kept low, for the purpose of which H₂O vapor and/or CO₂ can be used as a medium, advantageously H₂ O vapor isused. The CO₂ can be used only to a limited extent from the sulfitation.Consequently, after the condensation of the vapor, H₂ S can be obtainedat high concentrations for the muffle 13. If the stripping, in contrastto other processes, is carried out without CO₂ circulation withconcentrated CO₂, due to the very similar behavior of carbon dioxide andhydrogen sulfide, process control without splitting of bicarbonateduring the stripping is not possible. Partial splitting of thebicarbonate accelerates the substance transition. If the partialpressure of H₂ S decreases, as is the case with relatively highfractions of carbonate, this carbonate must again be converted intobicarbonate in the next intermediate fortification stage through theconversion with CO₂ in order to shift the partial pressure ratios againtoward increased partial pressures of H₂ S. This adsorption anddesorption is controlled alternatingly in several steps. In this way,not only the CO₂ fraction can be reduced but large saving of strippingvapor results and high H₂ S concentrations.

The reactions by absorption of CO₂ in the fortification stage fromcarbonate to bicarbonate is as follows:

Na₂ CO₃ +CO₂ +H₂ O →2 NaHCO₃

This CO₂ absorption is preferably carried out at increased pressures (>1bar) and low temperatures. If the carbon dioxide is to be absorbed atincreased temperatures, higher pressures are specifically required. As aCO₂ source, that carbon dioxide received from a decarbonation process intank 12 situated after the condensation of the vapor and that of thesulfitation is used.

With reference to FIG. 2, the intermediate fortification with CO₂ takesplace at increased pressure and increasing temperature. The pressure isgenerated via the geodetic gradient from the stripping stage to theintermediate fortification stage, and the process is carried from thepreceding stripping in the downward direction. The absorption of thecarbon dioxide is carried out in hydraulic condensers 18, 19, 29, 21(see FIGS. 2 and 3) in order to ensure sufficient dissolving reactionthrough a further pressure increase (approximately 2 bars absolute) andhigh dwelling time of the CO₂ in the liquid, with the CO₂ gas and thecarbonate, bicarbonate solution flowing in opposite directions (see FIG.3).

The bicarbonate solution saturated with CO₂ is drawn into the strippingstage due to its low pressure and further stripping reduces the sulfidecontent to values less than 1 g/l. The H₂ S stripping is carried out inbubble tray columns. The required stage number of more than about 15trays for each stripping stage represents low apparatus costs.

After separating the water vapor in condenser 17, the highlyconcentrated H₂ S gas from the stripping stages is supplied to the H₂ Smuffle 13 along with a regulated influx of air. In the muffle 13, thecombustion takes place automatically and the released waste heat can beutilized for further overheating of the saturation vapor generated inthe liquor burning boiler 1, which is only slightly overheated.

From the NaHSO₄ of the acidic flue gas washing process after the liquorburning boiler 1 and the SO₂ washing process after the H₂ S muffle 13,sodium sulfite is generated in a sulfitation unit 16 with thebicarbonate from the H₂ S stripping stage occurring in member 11 and CO₂is obtained in concentrated form. A regulated quantity of vapor isdirected into the sulfitation unit 16, A reaction occurring in thesulfitation unit 16 is:

NaHSO₃ +NaHCO₃ →Na₂ SO₃ +H_(2O+CO) ₂

That fraction of the bicarbonate after the H₂ S stripping stageoccurring in member 11, which is not required in the sulfitation unit16, is supplied to the decarbonation stage 12 after being heated toabout 115° C. The CO₂ vapor mixture is supplied to the H₂ S strippingstage 11. The Na₂ SO₃ is removed from sulfitation unit 16 and possiblyat least a portion thereof is directed to the first washing stage 4.

After these process steps, sodium sulfite and sodium carbonate areactively available as product in concentrations of up to 2.7 mol Na'/1for cellulose pulping or digestion.

If a further increase of the liquor strength is desired, such as wouldbe required, for example, in the case of preimpregnation, a higherliquor concentration can be achieved through crystallization ofbicarbonate and recycling of the mother liquor into the smelt dissolvingtank and dissolving of the crystallate in the sulfitation stage. Onedisadvantage though is a higher energy requirement and, for cooling ofthe crystallization stage, a greater cooling water requirement.

FIG. 2 depicts in a partial circuit diagram the H₂ S stripping stage 11in which, in several hydraulic condensers 18, 19, 20, 21, CO₂ is broughtinto solution at increasing pressure and increased temperature andallowed to react and between the stripping stages the fortification withCO₂ at a total pressure greater than one bar is carried out.

For further increasing the outward transfer of sulfate, the CaO orCa(OH)₂ is introduced into the combustion chamber of the liquor burningboiler 1 so that additionally calcium sulfate is generated which isdeposited as an undissolved sediment in the second washing stage 5,while the Na₂ SO₄ solution is added to the burning spent liquor.

The examples provided above are not meant to be exclusive. Many othervariations of the present invention would be obvious to those skilled inthe art, and are contemplated to be within the scope of the appendedclaims.

We claim:
 1. Process for converting spent liquor containing sodiumsulfite and carbonate with sulfate and thiosulfate impurities to sodiumsulfite and sodium carbonate-containing cooking liquor for an ASAMprocess or alkaline or acidic sodium sulfite process in which the spentliquor is burned in a liquor burning boiler with liquid slag extraction,said liquid slag extraction comprising Na₂ S or NaHS and CO₂ -containingdissolved sodium compounds and a multistage waste gas purification withrecovery of the sodium sulfur compounds, and waste gases leaving theliquor boiler are first freed of dust in a very dry process andsubsequently in at least one stage are washed with different washingliquids to form a purified waste gas, and the separated dust includingseparated Na₂ SO₄, is mixed with the liquor to be burned, and the liquidslag from the liquor burning boiler is dissolved in water and thedissolved sodium compounds including the Na₂ S formed, are converted bycarbonation with a portion of the purified waste gas to form NaHCO₃, Na₂CO₃ and NaHS and the dissolved gases H₂ S and CO₂, the improvementcomprising the steps of:stripping the H₂ S and CO₂ -containing dissolvedsodium compounds of the slag flowing from the carbonation stage with CO₂and water vapor in several stages such that after each stripping stage,CO₂ is absorbed at pressures of greater than 1 bar, and burning theremaining H₂ S gas after the condensation of water vapor in an H₂ Smuffle.
 2. The process of claim 1, further comprising the stepsof:washing the SO₂ and H₂ S-containing waste gases of the liquor burningboiler, freed of dust in a dry process, in a first washing stage at a pHof 6 to 7 with an Na₂ SO₃ to form NaHSO₃, washing the odor-intensive H₂S and mercaptan-containing waste gases flowing off from the firstwashing stage in a second washing stage at a pH greater than 7 to formNa₂ SO₄, recycling the Na₂ SO₄ to the spent liquor before it is burned,and directing the NaHSO₃ solution from said first waste gas washingstage of the liquor burning boiler to the SO₂ washing stage after themuffle, such that the bisulfite formed is subsequently mixed with thecarbonated sodium compounds at a pH greater than about 6 and the Na₂ SO₃formed is used for the liquor as well as for the washing liquid of thefirst washing stage.
 3. The process of claim 2, wherein theodor-intensive H₂ S and mercaptan-containing waste gases flowing offfrom the first washing stage are washed in the second washing stage withH₂ O₂.
 4. The process of claim 1, further comprising the stepsof:stripping the dissolved sodium compounds from the smelt in severalstages by CO₂ -containing gases setting free H₂ S, and providing abicarbonate splitting stage connected to a last one of said strippingstages to direct CO₂ to said last stripping stage such that the endproduct is Na₂ CO₃ containing only low levels of NaHCO₃ and H₂ Simpurities and is suitable for causticizing in order to obtain NaOH. 5.The process of claim 1, wherein the stripping of the H₂ S in severalstages is carried out at increasing total pressure and increasingtemperature.
 6. The process of claim 1, further comprising the step ofintroducing additional CO₂ to form bicarbonate at an increased totalpressure between any two of said several stripping stages.
 7. Theprocess of claim 1, further comprising the steps of:introducing CaO andCaO(OH)₂ in dust form into the boiler to react with SO₃, to form CaSO₄,utilizing the resulting NaSO₄ and CaSO₄ -containing ash from the drydust-removal stage with the unreacted calcium compounds in the secondwashing stage, separating the insoluble calcium compound, and utilizingthe concentrated NaOH, Na₂ SO₄ solution in the second washing stage andthe consumed washing solution is supplied to the spent liquor to beburned.
 8. The process of claim 7, further comprising the stepsof:separating flue ash from the liquor burning boiler in anelectrostatic filter and suspending the flue ash with water, and addingsodium carbonate in a clarifier for the complete precipitation of thecalcium at pH-values greater than 6 such that insoluble calciumcompounds are separated and the thus purified sodium sulfate solution isagain applied to the spent liquor before the mix tank.
 9. The process ofclaim 7, further comprising the steps of:separating the resulting flueash in an electrostatic filter and suspending the flue ash in water andat pH-values greater than 7, and washing the flue gas with the additionof H₂ O₂, whereby insoluble calcium compounds are separated in the formof calcium sulfate and calcium and the thus purified sodium sulfatesolution is again supplied to the spent liquor before the mixing tank.10. The process of claim 9, further comprising the steps of:convertingthe calcium fractions with SO₂ at pH-values greater than 5.5 to formcalcium sulfate in a flue gas washing stage preceding the alkalinewashing stage, and separating the calcium sulfate thus-formed from thesodium sulfate solution and supplying the solution thus-formed to thespent liquor.
 11. A process for converting spent liquor containingsodium sulfite and sodium carbonate with sulfate and thiosulfateimpurities into cooking liquor containing sodium sulfite and sodiumcarbonate, said method comprising the steps of:burning said spent liquorin a boiler; extracting liquid slag from said boiler, said liquid slagcomprising Na₂ S or NaHS, and CO₂ -containing dissolved sodiumcompounds; extracting waste gases from said boiler; removing dust fromsaid waste gases in a dry process, said dust comprising Na₂ SO₄ ;washing said waste gases to form purified waste gases; mixing said Na₂SO₄ with said spent liquor; dissolving said liquid slag in water to formdissolved H₂ S and CO₂ -containing sodium compounds; stripping said H₂ Sand CO₂ -containing dissolved sodium compounds with CO₂ and water vaporin several stages such that after each stripping stage, CO₂ is absorbedat pressures of greater than 1 bar, to form NaHCO₃, Na₂ CO₃ and NaHS andthe dissolved gas H₂ S; and burning the remaining H₂ S gas after thecondensation of water vapor in an H₂ S muffle.